1. Field of the Invention
This invention concerns a scanning optical system for a laser printer or other device wherein a light beam is made to scan a scanning surface by using a scanning deflector, and in particular concerns the prevention of ghost images.
2. Description of the Prior Art
In conventional scanning optical systems that use a polygon mirror as a deflector, a light beam is first brought to an image in an auxiliary scanning plane and forms a spot on a scanning surface using an anamorphic f.theta. lens in order to correct a tilting error of the reflecting surface of the polygon mirror.
In such a conventional optical scanning device, as the optical system has a high positive power in the auxiliary scanning direction, the peripheral part of the image in that direction is, to some extent, curved behind the scanning surface. This curvature has to be corrected by the displacement of the reflection point on the polygon mirror when the mirror rotates.
In this conventional scanning system, however, the angle made by the light beam incident on the polygon mirror with the optic axis of the f.theta. lens is generally 50.degree.-90.degree.. In this case the displacement of the reflection point on the polygon mirror and hence the curvature of the image are both asymmetrical with respect to the optic axis, and if the lens used is symmetrical with respect to the optic axis, the above asymmetrical curvature cannot be corrected. If the number of reflecting surfaces of the polygon mirror is the same, the displacement of the reflection point on the mirror depends on the mirror diameter. The diameter of the polygon mirror can therefore be increased so as to correct for the curvature of the image, but in this case the aforesaid asymmetry also increases.